As the telecommunication systems have evolved, services utilizing multimedia transmission have gained more and more ground. IP multimedia core network subsystem of the 3rd Generation Partnership Project (3GPP) has been designed to enable operators a standardized way to offer their subscribers multimedia services based on and built upon Internet applications, services and protocols. Smooth integration of existing services and stand-alone network elements for the standardized IP multimedia core network subsystem thus seems to present an inevitable task for most operators.
For example, Push-to-talk over Cellular (PoC) service can be provided as a packet-based user or application level service in a digital communication system. In PoC, the underlying communications system provides the basic connections (i.e. IP connections) between the communications applications in the user terminals and the communication service. The PoC communication service can be provided by a communication server system while the client applications reside in the user equipment or user terminals.
Owing to the significant interest for the PoC services, individual vendors have already provided early adaptations of the technology, primarily in form of stand-alone PoC systems. Quite recently a group of interested organizations have prepared an industry specification for PoC, with the aim to follow existing 3rd Generation Partnership Project (3GPP) IP Multimedia Subsystem (IMS) specifications. The standardization work in this direction has since then continued in Open Mobile Alliance (OMA) using this industry specification as a starting point.
However, integration of the stand-alone PoC systems to IMS systems has presented some unexpected problems. When a PoC operator wishes to integrate his stand-alone PoC system in the IMS, it would seem obvious to create an IMS subscription for all identities and subscribers of the existing PoC system, but this is, however, not the case. For example, all existing PoC users would then be obliged to upgrade their current PoC terminals into IMS capable PoC terminals and to configure the new terminals. At the time of integration the installed base of the existing system is likely to be considerable, and such extensive operations could meet a lot of resistance and cause a considerable amount of administrative work.
Furthermore, it is not likely that all PoC subscribers immediately wish to become IMS subscribers. Some subscribers may be ready for direct IMS migration, but some subscribers may also wish to time the migration to a later date and to control the timing themselves. Users who do not wish to become IMS subscribers are not willing to be charged for any IMS traffic, either. However, the implementation of two kinds of IMS traffic, chargeable and non-chargeable, is not easily managed.
Conventionally subscribers are categorized according to type by arranging the users' identity information into appropriate ranges. This may be implemented, for example, by configuring identities of the system so that the type of each user is easily deducible from the identity information, or by mapping the identities according to type to some other pre-categorized identity information. However, when the status of the categorizing type is not static but changes dynamically, these conventional means easily lead to complicated and conflicting definitions and implementations.
For example, users of the existing PoC system that do not migrate at integration may be initially arranged to a specific addressable identity range. When they begin to show an interest in IMS services and wish to become IMS subscribers, they somehow need to be excluded from the address range. In order to manage this, the identity ranges would need to be divided which would make the management of the subscriber information very complicated. Such complexity would also affect to the critical activities based on identity information, especially the routing where the next hop is typically defined based on the identity information in the transmitted message.